Method of evaluating the wear of engine oil taking into account the addition of fresh oil

ABSTRACT

A method of evaluating engine oil wear on the basis of viscosity measurement, taking into account the addition of fresh oil is described.  
     First, using the oil viscosity sensor, the amount of oil added in the addition of fresh oil is detected, then the oil viscosity sensor determines the viscosity of the oil mixture (η 2 ) of the engine oil and fresh oil; the quantity (V n ) of fresh oil added is determined in the electronic analyzer from the difference between the old oil level, i.e., the oil volume (V 1 ), and the new oil level, i.e., oil volume (V 2 ); and the viscosity (η n ) of the fresh oil is determined from the viscosity (η 1 ) of the old engine oil immediately before the point in time of the addition of fresh oil and from the viscosity (η 2 ) after the addition; and a hypothetical initial viscosity (η′ 0 ) is calculated as a new corrected reference value from the known value of the viscosity (η 0 ) of the old engine oil in the new condition, the quantity (V 1 ) of the engine oil still present at the point in time of the addition, the calculated viscosity (η n ) of the fresh oil and the calculated quantity (V n ) of the fresh oil.

[0001] The present invention relates to a method of evaluating the wear on engine oil, taking into account the addition of fresh oil, having the features specified in the preamble of claim 1.

BACKGROUND INFORMATION

[0002] Software-based systems for displaying the information that it is time to change the engine oil in a motor vehicle are known, some of them based on algorithms which analyze parameters such as the distance traveled since the last oil change or the frequency of cold starts. In addition, other known methods rely on sensor signals which directly describe the physical condition of the oil, e.g., the dielectric constant of the oil or, as a much more reliable variable, the viscosity (η) of the oil. A viscosity-based oil change criterion is derived from recording the change in viscosity since the last engine oil change, because wear on engine oil is usually associated with an increase in viscosity. For example, in a simple case, a viscosity value (η_(max)) may be defined as a limit value which must not exceed the viscosity value (η₀) of the fresh oil as a reference value by more than a certain percentage, e.g., no more than 30%. The viscosity threshold above which engine oil should be changed and which is displayed for the driver of the vehicle is then given by the following equation, for example:

η_(max) =k×η ₀   (1)

[0003] Factor k defines the allowed increase in viscosity, which corresponds to k=1.3 with a 30% allowed increase.

[0004] Adding fresh oil of a different type or a different viscosity class is problematical with these methods and similar methods of evaluating engine oil wear on the basis of viscosity measurements. Oil added in addition then has a different fresh oil viscosity, so the oil change criterion is subject to a certain distortion. For example, the addition of engine oil of a higher viscosity class would simulate aging, although a de facto improvement in oil condition is achieved by this addition of oil. Conversely, the addition of a lower-viscosity engine oil would simulate an improvement in the condition of the oil to an extent which does not conform to the actual condition. The fresh oil viscosity values for engine oil from adjacent viscosity classes typically differ by 30%.

ADVANTAGES OF THE INVENTION

[0005] The method according to the present invention offers the advantage over the related art that it is possible to evaluate engine oil wear on the basis of viscosity measurements, taking into account the addition of fresh oil, and this then permits a correction of the reference value.

[0006] Specifically, the amount of oil added in topping off with fresh oil is detected by the oil level sensor, then the viscosity of the oil mixture (η₂) of engine oil and fresh oil is determined by the oil viscosity sensor after an operating time and/or after reaching a predetermined oil temperature, the quantity (V_(n)) of the added fresh oil is determined by the electronic analyzer from the difference between the old oil level, i.e., oil volume (V₁) and the new oil level, i.e., oil volume (V₂), and the viscosity (η_(n)) of the fresh oil is determined from the viscosity (η₁) of the old engine oil immediately before the point in time of adding more oil and from the viscosity (η₂) after the addition, and a hypothetical initial viscosity (η′₀) is calculated as a new corrected reference value from the known value of the viscosity (η₀) of the old engine oil in the new condition, the quantity (V₁) of the engine oil still present at the point in time of adding the new oil, the calculated viscosity (η_(n)) of the fresh oil and the calculated quantity (V_(n)) of the fresh oil.

[0007] A new limit value (η′_(max)) for the viscosity threshold of the oil mixture is calculated from the corrected reference value, so that the oil should be changed after this limit value, which replaces the original value from (1), e.g., through the following computation rule:

η′_(max) =k×η′ ₀   (2)

[0008] To determine the viscosity (η) of the added fresh oil and to determine the hypothetical initial viscosity (η′₀) as a new corrected reference value, a simple mixing rule is used to advantage, where the value sought is a simple function of the known viscosities and the different oil quantities.

[0009] According to another preferred embodiment of this method, additional parameters for the aging of oil may be taken into account as a viscosity threshold by the connected electronic analyzer in determination of the limit value of the maximum allowed viscosity (η′_(max)), e.g., the temperature gradient of heating of the oil, the number of cold starts or the length of trips traveled, and/or the operating times.

[0010] The determination of the corrected reference value (η′₀) may be made more accurate by taking into account additional relationships and evaluation criteria and may even be stored as a reference characteristic in the electronic analyzer. For example, the temperature dependence of the viscosity may also be taken into account in the evaluation criteria.

[0011] To increase the accuracy of the reference value, additional known evaluation methods may also be taken into account in evaluating engine oil wear, so that the engine oil quality of a vehicle may never fall below a required level without drawing the attention of the driver to an urgent need for an engine oil change.

DRAWINGS

[0012] This method is described in greater detail below on the basis of two figures.

[0013]FIG. 1 shows a coordinate system representing the decrease in the oil volume over the operating time or the distance traveled by a motor vehicle, and

[0014]FIG. 2 shows a coordinate system which illustrates the typical increase in viscosity over the operating time of an engine or distance traveled.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0015]FIG. 1 shows the typical change in oil volume (V₀) from the point in time of the last oil change until the point in time (1) of the addition of fresh oil (V₁), after which the oil volume increases again to a level (V₂), the various quantities, i.e., oil volumes, being determined as a function of the filling level via the level sensor and the electronic analyzer.

[0016]FIG. 2, in contrast, illustrates the increase in viscosity of an engine oil starting from a viscosity (η₀), when the engine oil is new, up to a viscosity (η₂) at the point in time (2) when oil is added. The addition of fresh oil then results in a sudden change in the value of the viscosity (η₂) which characterizes the mixture of oil which is already partially “used” and freshly added oil. For the illustration in FIG. 2, it has been assumed that the resulting viscosity value (η₂) drops when fresh oil is added, although that need not necessarily be the case if engine oil of a higher viscosity class than the oil used in the last oil change is added. The viscosity value (η₂) is not determined immediately after the addition of oil because of incomplete mixing, but instead is established as a steady-state measured value only after a certain engine operating time and after thorough mixing of the engine oils to form a homogeneous mixture.

[0017] The quantity of fresh oil (V_(n)) added is determined from the measured oil level, i.e., the volume derived therefrom:

V _(n) =V ₂ −V ₁.

[0018] A mixing rule is used to determine the viscosity (η_(n)) of the engine oil added, which is not known a priori. Such mixing rules are formulated on the basis of scientific models for the respective materials in question, and they describe the resulting viscosity for a mixture of two fluids. In general, the viscosity of the mixture of the fresh oil added and the oil already present is obtained as a function of the viscosity (η₁) of the oil already present, its volume fraction (V₁), the viscosity (η_(n)) of the fresh oil added and its volume (V_(n)). A very simple model for such a mixing rule is based on the assumption that the viscosities of the mixed oils are weighted with the corresponding relative volume fractions:

η₂=η₁(V ₁/(V ₁ +V _(n)))+η_(n)(V _(n)/(V ₁ +V _(n)))

[0019] By rearranging, the viscosity (η_(n)) of the added fresh oil may be calculated from the known variables V₁, η₁, V_(n) and η₂. Using the mixing rule again, the hypothetical viscosity (η′₀) is determined which would be obtained by mixing the quantity (V_(n)) of fresh oil added with the last oil change having the viscosity (η₀) and the quantity (V_(n)) of fresh oil of the viscosity (η_(n)) added. New limit value η′_(max) is determined from this new corrected reference value, e.g., by calculating it by multiplying by factor k for the oil mixture, which then replaces the original value (1).

[0020] The absolute limit value for the acceptability of a viscosity, however, may also be formulated more generally as a function in which the initial viscosity (η₀) of the fresh oil or the calculated hypothetical initial viscosity (η′₀) of the oil mixture is taken into account in addition to other parameters for the oil change recommendation. The change in viscosity in comparison with the reference value will in the general case enter into a more complex algorithm which takes into account other characteristic values.

[0021] The method of evaluating engine oil wear on the basis of viscosity measurements, taking into account the addition of fresh oil, is thus performed by characterizing the oil mixture by using a limit value representing the fresh oil. In the addition of oil, a new corrected reference value for the resulting oil mixture is determined from the quantity of oil added and the resulting mixture viscosity, taking into account the partial wear of the oil already present in the engine. The reference value may also be a reference characteristic, e.g., if the temperature dependence of the viscosity is also taken into account. The viscosity-based evaluation of wear may also be just one of several aspects of a more general evaluation algorithm for evaluating the condition of the engine oil. 

What is claimed is:
 1. A method of evaluating engine oil wear on the basis of viscosity measurements, taking into account the addition of fresh oil using an oil viscosity sensor, an oil level sensor and a connected electronic analyzer having a display device, the aging of the engine oil and an oil change recommendation resulting therefrom being determined as a reference value by evaluation of the change in the prevailing oil viscosity at at least one temperature, in comparison with the viscosity of the fresh oil added at the last oil change, wherein the quantity of oil used in the addition of fresh oil is detected by the oil level sensor; the viscosity of the oil mixture (η₂) of aged engine oil and fresh oil is determined together with the temperature by the oil viscosity sensor at at least one temperature value; the quantity (V_(n)) of fresh oil added [is determined] in the electronic analyzer from the difference between the old oil level or oil volume (V₁), and the new oil level or oil volume (V₂) ; the viscosity (η_(n)) of the fresh oil is determined from the viscosity (η₁) of the old engine oil immediately before the addition of fresh oil and from the viscosity (η₂) immediately after the addition, and a hypothetical initial viscosity (η′₀) is calculated as a new corrected reference value from the known value of the viscosity (η₀) of the old engine oil when new, the quantity (V₁) of the engine oil still present at the point in time of the addition, the calculated viscosity (η_(n)) of the fresh oil, and the calculated quantity (V_(n)) of fresh oil.
 2. The method as recited in claim 1, wherein a limit value of the viscosity (η_(max)) is determined from the initial viscosity (η₀) by multiplying the initial viscosity by a factor (k), so that the display of the end of the oil change interval by the display device is triggered when this limit value is exceeded by the viscosity measured at a given time at a corresponding temperature value, and when adding fresh oil, the new limit value (η′_(max)) is obtained from the calculated new initial viscosity (η′₀) by multiplying the initial viscosity by the factor (k) of the admissible increase in viscosity.
 3. The method as recited in claim 2, wherein the factor has a value greater than
 1. 4. The method as recited in one of the two preceding claims, wherein a mixing rule is used to determine the viscosity (η_(n)) of the fresh oil and/or to determine the hypothetical initial viscosity (η₀) of the oil mixture when new as the new corrected reference value.
 5. The method as recited in one of the preceding claims, wherein indirect influencing factors on aging of engine oil are taken into account in determination of the oil change interval.
 6. The method as recited in claim 5, wherein oil heating temperature gradients are taken into account as an indirect influencing factor on the aging of the engine oil.
 7. The method as recited in claim 5, wherein the number of kilometers driven and/or the number of hours of operation since the last oil change are taken into account as indirect influencing factors on the aging of the engine oil.
 8. The method as recited in claim 5, wherein the rotational speed oil temperature characteristics are taken into account as indirect influencing factors on the aging of the engine oil.
 9. The method as recited in claim 5, wherein the dielectric constant (permittivity) of the engine oil is taken into account as an indirect influencing factor on the aging of the engine oil.
 10. The method as recited in claim 5, wherein the conductivity of the engine oil is taken into account as an indirect influencing factor on the aging of the engine oil.
 11. The method as recited in claim 5, wherein the frequency of cold starts and/[or] the load on the vehicle are taken into account as indirect influencing factors on the aging of the engine oil.
 12. The method as recited in one of the preceding claims, wherein the corrected reference value (η′₀) is formed by taking into account additional measured relationships as the reference characteristic.
 13. The method as recited in claim 12, wherein the reference characteristic is a viscosity/temperature characteristic, ranges not measured being supplemented by interpolation/extrapolation methods. 